The present invention relates to an apparatus which enables a coordinate positioning machine (such as a machine tool) to determine the position of an object relative to a reference point. It may, for example, be employed on a machine tool for toolsetting operations.
A known tool setting device for use on a machine tool includes a light source which generates a fine beam of light which is incident upon a detector. During a toolsetting operation, the machine is operated to move the tool in a direction transverse to the direction of propagation of the light beam until a part of the tool interrupts passage of the light beam. Detection of this interruption results in the generation of a trigger signal in the detecting unit, which is used by the machine to establish the relative position of its moving parts in order to determine dimensions of the tool. Such devices are known, for example, from German Patent Nos. DE 42 385 04 and DE 42 448 69, French Patent No. 2,343,555, European Patent No. 98,930 and U.S. Pat. No. 4,518,257. The devices may be used additionally for measuring the length or diameter of a tool to monitor tool breakage or wear.
The devices disclosed in the above-mentioned patent specifications use a narrow light beam into or through which the tool is passed. The detection units detect when the tool breaks into the beam from the resulting drop in the intensity of the light falling on them.
The accuracy with which these devices can measure tool position is dependent on the diameter of the laser beam, the smaller the diameter the more accurate the measurement.
For example, French Patent No. 2,343,555 describes a system in which the laser produces a coherent beam the diameter of which is on the order of 0.7 to 0.8 mm. European Patent No. 98,930 proposes the use of a laser light source because lasers supply a sharply defined light beam for high measurement accuracy. U.S. Pat. No. 4,518,257 describes a system in which the laser beam is focused to a small examination zone at which all measurements are made.
One problem with all of these devices is that the smaller the laser beam, the more difficult is the task of aligning the laser beam with the detector system.
Another problem with the prior art systems exemplified in European Patent No. 98,930 is that the optical system has to be kept clean to avoid build up of contaminants on the optical components which can reduce the intensity of light received by the receiver and cause the signal from the detector to be emitted early. This is particularly so in the case of devices which rely on the reduction in intensity produced by the smallest obstruction of the beam to produce a measurement signal.
European Patent No. 98,930 describes two methods of keeping the optical system clean, one of which involves directing an air blast at the external glass faces of the housings of the emitter and receiver, the other of which involves the use of movable mechanical shutter. These methods require the provision of additional cost and complexity to the devices.
A problem with a focused system, as in U.S. Pat. No. 4,518,257, is that the tool can only be measured in the small examination zone. Unfocused systems do not suffer from this problem, but the beam must be accurately aligned with an axis of travel of the machine tool to permit accurate measurements anywhere along the beam path. Prior unfocused systems have been awkward to align, because each time an adjustment is made to the direction of the beam path, the position of the beam must be checked at two different positions along the path, followed by further adjustment as necessary.
One tool setting device known as the Tool Verification Gage (TVG) used by General Electric in the USA has an aperture in a plate at a light transmitter and a similarly apertured plate in a light receiver. The aperture is in the order of 1 mm in diameter. The device operates in the same manner as the devices described previously. It appears that the device is supplied with pressurised air such that the air is caused to exit from the apertures. In this manner the apertures are kept free from contamination and light can pass through the apertures unhindered.
However, in that device the light beam is exposed to air turbulence in the apertures and this causes irregularities at the detector in the form of signal noise. Additionally the aperture allows a high volume of air to escape and so the device is costly to operate. Again lenses are employed to collimate the beam or light from the transmitter and therefore setting up the device is not easy because alignment of the fine beams is required.
The present invention provides alternative devices which are suitable for toolsetting on a machine tool (or other applications on other coordinate positioning machines). The devices include a number of technical aspects in which they differ from the prior art devices discussed above and which enable various ones of the problems which arise with those prior art devices to be eliminated or reduced.
According to a first aspect of the present invention there is provided a position determination device for a machine utilising coordinate positioning, the device comprising a light emitting unit and a light detecting unit, the light emitting unit having a housing within which a light source is housed for emitting a light beam which in use propagates towards the light detecting unit which in turn includes a housing having a light channel located in register with the light beam, and a light detecting device located substantially in register with the light channel thereby to detect the light beam propagating from the light emitting unit, wherein the light beam is uncollimated such that the size of the resultant spot formed on the detecting unit is substantially larger than the light channel at the light detecting unit.
The light emitting unit may have a light channel also and the or each light channel may be an aperture. Where the light channel is an aperture one or both of the housings may include this aperture in the form of a conduit through which pressurised air supplied to the housings may pass from the interior of the housing to the exterior. In a preferred embodiment of this aspect of the invention the conduit extends transverse or obliquely to the direction of propagation of the light beam between an interior and an exterior aperture of the housing. Also preferably the interior and exterior apertures overlap to the extent that light may pass through the conduit so that the conduit also forms the aperture through which the light beam leaves or enters the respective housing.
Thus the housings of the light emitting and/or detecting units are constructed amongst other things for the purpose of preventing ingress of dirt or contaminants.
The light channels are formed from any light transmissive medium surrounded by a non-transmissive medium, e.g. an aperture or window in a non-transmissive material.
Such a channel has preferably a cross-sectional area of 0.8 mm2 or less. More preferably a channel has at its narrowest point a cross-sectional area in the range of 0.02 mm2 to 0.8 mm2. Where a channel is approximately circular such areas would provide a diameter between 50 microns and 1 mm. Small apertures of this size are often referred to as pinholes.
According to a second aspect of the present invention there is provided a position determination device for a machine utilising coordinate positioning, the device comprising a light emitting unit and a light detecting unit, the light emitting unit having a housing within which a light source is situated for emitting a light beam which in use propagates towards the light detecting unit which in turn includes a housing and a light detecting device to detect the light beam propagating from the light emitting unit, wherein one or both of the housings include a light channel in the form of a conduit through which pressurised air in use supplied to the or each housing may pass from the interior of the housing(s) to the exterior thereof, and the or each conduit extends transversely or obliquely to the general direction of propagation of the light beam.
Preferably the light beam is uncollimated such that the size of the resultant spot formed on the detecting unit is substantially larger than the aperture within the light detecting unit.
A third aspect of the present invention provides a device which enables determination of position of an object on a coordinate positioning machine, including a light emitting unit and a detecting unit, the light emitting unit including a light source and an adjuster for adjusting the angle of a light beam produced by the light source, wherein the adjuster has associated therewith a common centre point, located between the light emitting unit and the detecting unit, that the adjuster directs the light beam through said centre point, and that when the alignment of the beam is adjusted by the adjuster the beam continues to pass through said centre point. This may be achieved by providing the adjuster with a curved bearing surface, preferably a spherical bearing surface, centred on said common point.
In this third aspect of the invention, the alignment of the beam may be simplified since it is no longer necessary to measure the position of the beam repeatedly at two different points along the beam path. Rather, a measurement can be made just once at the common centre point, followed by repeated measurements after each adjustment at another single point along the path. According to a fourth aspect of the present invention a device which enables determination of position of an object on a coordinate positioning machine includes a light emitting unit and a detecting unit, the light emitting unit having a housing within which a light source is situated, the housing having a first light channel through which a beam of light from the light source passes, and which propagates towards the light detecting unit which in turn includes a housing having a second light channel located in register with the light beam, and a light detecting device located substantially in register with the light channels thereby to detect the light beam propagating from the light emitting unit, wherein the light beam is uncollimated and the detecting unit is arranged to emit a trigger signal when the light level it receives has fallen to one half of that which it receives when the beam is not occluded.